The present invention relates to the slurry polymerization of olefin monomers. More particularly, the present invention relates to improved techniques for continuously withdrawing a portion of the fluid slurry from a loop reaction zone and operating a loop reactor having a plurality of continuous take off.
Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, feed materials such as diluent, monomer and catalyst are introduced to a loop reaction zone, and a fluid slurry containing solid polyolefin particles in a liquid medium (usually an inert diluent and/or unreacted monomer) is circulated through the loop reaction zone. A portion of the fluid slurry is withdrawn from or taken off the reaction zone so that the solid polyolefin particles can be recovered.
In continuous loop reactors, the various feed materials may be introduced to the loop reaction zone in various ways. For example, the monomer and catalyst may be mixed with varying amounts of diluent prior to introduction to the loop reaction zone. The monomer may also be combined with recycled diluent and then fed to the loop reaction zone. In the loop reaction zone, the monomer and catalyst become dispersed in the fluid slurry. As the fluid slurry circulates through the loop reaction zone, the monomer reacts at the catalyst in a polymerization reaction. The polymerization reaction yields solid polyolefin particles in the fluid slurry.
Slurry polymerization in a loop reaction zone has proven commercially successful. The slurry polymerization technique has enjoyed international success with billions of pounds of olefin polymers being so produced annually. With this success has come the desirability, in some situations, of building larger reactors. Larger reactors lead to higher flow rates of fluid slurry. The flow rate inside a loop reactor can be as high as 1,000,000 gallons (3,785,410 liters) per minute or more.
In a continuous take off process, the withdrawn slurry is usually a small portion of the fluid slurry that is in a loop reaction zone. The flow of this smaller withdrawn slurry typically ranges from 50 gallons (189 liters) per minute to 3000 gallons (11,356 liters) per minute. The large flow in the reactor can transport polymer in the form of slurry particles that are small, but also transport larger polymer particles or fused chunks of polymer. The larger polymer chunks or particles, some with diameters larger than the opening of the take off valve, may plug the take off valve. When such larger particles attempt to pass through the take off valve, either the particle breaks or the take off valve is restricted in flow.
Flow restriction causes loss of flow through the take off valve and may cause more polymer particles to build up. This may cause the reactor pressure to increase, since it is usually controlled (at least partially) by how much the take off valve is opened. If the build up in polymer particles is quicker than the action of the control mechanism for controlling pressure by opening the take off valve, a plugged line and excessive reactor pressures may result. This may be especially severe for fused or atypical polymer chunks that can grow in the loop reactor and have a much larger dimension than the largest polymer particle size. Plugged reactor take off valves can lead to reactor over pressure, downtime, production loss, and in extreme situations, relief of reactor pressure by process safety relief valves.
A ram valve may be used to close off a continuous take off mechanism that is not being used. The ram valve has the advantage of preventing polymer accumulation in the slurry withdrawal line. However, if one desires to begin using that continuous take off mechanism, it takes some time (for example, 10 to 20 minutes) to make it ready for operation, and the ram valve must be manually opened.
As one aspect, a polymerization process is provided. The process includes feeding a feed material comprising at least one olefin monomer to a loop reaction zone and polymerizing the olefin monomer to produce a fluid slurry comprising solid olefin polymer particles. The process also includes continuously withdrawing a portion of the fluid slurry through a plurality of active continuous take off, and passing the withdrawn slurry portion through a take off valve. The process may also include monitoring the pressure of the feed material and adjusting the take off valve in response to the monitored feed material pressure. The process may also include flushing diluent through least one inactive continuous take off. The inactive continuous take off may be activated when one of the active continuous take off is at least partially plugged. The process may also include sensing when the take off valve is closed and automatically flushing a slurry withdrawal line associated with the take off valve with diluent when the take off valve is closed.
As another aspect, a loop reactor apparatus is provided. The loop reactor apparatus includes a plurality of major segments and a plurality of minor segments. Each of the major segments is connected at a first end to one of the minor segments, and is connected at a second end to another minor segment. As a result, the major segments and the minor segments form a continuous flow path adapted to convey a fluid slurry. The continuous flow path is substantially free from internal obstructions. The loop reactor also includes a means for introducing an olefin monomer and/or a liquid medium (for example, an inert diluent) into the continuous flow path, a means for introducing a polymerization catalyst into the continuous flow path, and at least two means for continuously taking off a portion of the fluid slurry from the continuous flow path. Alternatively, the loop reactor includes at least four means for continuously taking off a portion of the fluid slurry. Such a loop reactor may have a volume greater than 30,000 gallons, alternatively greater than 35,000 gallons, alternatively greater than 40,000 gallons, alternatively greater than 45,000 gallons, alternatively greater than 50,000 gallons, alternatively greater than 75,000 gallons, alternatively greater than 100,000 gallons.
As yet another aspect, a loop reactor apparatus is provided. The loop reactor apparatus comprises a plurality of major legs and a plurality of minor segments. Each minor segment connects two of the major legs to each other, and by these connections, the legs and the segments comprise a continuous flow path. A monomer feed is attached to one of the legs or segments. At least one catalyst feed is attached to one of the legs or segments. A continuous take off is attached to one of the legs or segments. The continuous take off includes a slurry withdrawal line in fluid communication with the reactor, a take off valve disposed along the slurry withdrawal line for regulating flow of the slurry through the slurry withdrawal line, and a flush line fluidly connected to provide diluent to the slurry withdrawal line.
In the foregoing loop reactor apparatus, at least two of the minor segments form continual curves. At least two of the continual curves have one or more continuous take off mechanisms or means attached to them. The loop reactor apparatus may be essentially free of horizontal flow paths in that all the major legs are connected by continual curves.
The apparatus may also include at least one spare continuous take off mechanism or means for continuously withdrawing product slurry. Preferably, the continuous take off mechanism or means comprises a V-ball valve having a nominal body size of at least 1xc2xd inch. The continuous take off valve may be automatically controlled by a controller, which adjusts the continuous take off valve in response to one or more input signals from pressure transmitters on the monomer feed to the loop reactor (or other means for introducing the olefin monomer to the loop reactor). Additionally or alternatively, a pressure transmitter may be disposed on the slurry withdrawal line downstream of the continuous take off valve. The pressure transmitter may be operatively connected to provide a signal to the controller. One or all of the continuous take off mechanisms or means may be automatically controlled in such fashions.
The continuous take off mechanisms or means may be the exclusive means for withdrawing the portion of slurry; that is, the present process and apparatus allow the elimination of settling legs from the reactor altogether.
As yet another aspect, a continuous take off mechanism for a loop polymerization reactor is provided. The mechanism comprises a slurry withdrawal line in fluid communication with the reactor, a take off valve disposed along the slurry withdrawal line for regulating flow of the slurry through the slurry withdrawal line, a flush line fluidly connected to provide diluent to the slurry withdrawal line, and a controller. The controller is configured to receive an input signal from pressure transmitters disposed on a monomer feed and on the slurry withdrawal line downstream of the take off valve. The controller is also configured to send an output signal to adjust the take off valve.
As yet another aspect, a process for starting a loop polymerization reactor is provided. The process comprises feeding ethylene to the reactor and feeding isobutane to the reactor. The mass ratio of ethylene to isobutane fed to the reactor is sufficiently low to avoid plugging of the continuous take off mechanism.
As still another aspect, a process for operating a loop polymerization reactor is provided. The process includes feeding ethylene to the reactor, feeding isobutane to the reactor, feeding a polymerization catalyst to the reactor, circulating a fluid slurry comprising unreacted ethylene and solid polyethylene particles in the isobutane through the reactor, continuously withdrawing a portion of the fluid slurry through a plurality of continuous take off mechanisms, and continuously flushing isobutane through the slurry withdrawal line of an inactive continuous take off mechanism. In this manner, a xe2x80x9chot sparexe2x80x9d or xe2x80x9chot standbyxe2x80x9d is provided for the reactor.